Depression occurs in over 10% of the US population each year, and women are reported to experience depression twice as frequently as men. Physiological reactivity to social stressors is evolutionarily conserved and both rodents and humans experience social conflicts that can alter affect and behavior. In animal models social stress induces anhedonia, a loss of interest in previously rewarding activates, which is a major diagnostic feature of depression. Diminished reward can result in reduced consumption of sucrose and reduced time engaging in social interaction. This allows for the study of how changes in the brain are associated with these changes in behavior. Aggressive interactions are infrequent in commonly used laboratory rats and mice, making it difficult or impossible to study effects of social stress on females. California mouse (Peromyscus californicus) males and females are territorial, making them suitable for studying sex differences. Female but not male California mice exposed to social stress spend less time engaging in social interactions. Oxytocin (OT) and arginine vasopressin (AVP) are related neuroproteins that play important roles in social interactions. Both proteins have been identified as potential targets for drugs in the treatment of depression. The proposed study uses three experiments to examine sex differences in how OT and AVP systems in the brain respond to social stress. In the first experiment, triple-label immunohistochemistry will be used on brain tissue acquired from mice exposed to social stress, social interaction, or control conditions. This allows for quantifying the number of OT and AVP neurons that contain c-fos, a protein whose presence suggests that the cells were recently activated. The second experiment will use real time PCR to examine how OT and AVP receptor mRNA expression in the lateral septum, central amygdala, paraventricular nucleus and pituitary is changes 4 wk after social defeat. Finally, in experiment 3, I will infuse OT or OT receptor antagonist into the lateral ventricles just before chronic social stress to determine whether manipulating OT function can block or induce the development of social aversion 4 wk later. Together these experiments should provide insight into whether OT and AVP play a role in the development and expression of social withdrawal.